The short answer is yes! If you change the settings, it will reset them for all users in the course, regardless of how they got there.
Completion settings update when cron runs - this is set at the server level. There is a scheduled task option in your admin settings that will tell you when each task runs - this is what is meant by the syncing..
Completion settings update when cron runs - this is set at the server level. There is a scheduled task option in your admin settings that will tell you when each task runs - this is what is meant by the syncing..